1. Field of the Invention
This invention relates to an active broad-band reception antenna having a passive antenna component with a frequency-dependent effective length. The output connections of this component are connected to the input connections of an amplifier circuit. Electrically long antennas, or antennas directly coupled to electrically large bodies, when excited with an electric field strength that is kept constant over the frequency, have a frequency-dependent no-load (or open-circuit) voltage that is defined by their effective length le(f) In particular, in the frequency range above 30 MHz, the antenna noise temperature TA, coming from low frequencies, has dropped in a terrestrial environment so that for bipolar transistors, a source impedance, near the optimum impedance Zopt has to be specified for noise adaptation on the side of the passive component of the antenna, so as to avoid incurring any significant loss of sensitivity due to the noise of the transistor. The basic form of an active antenna of this type is known, for example from DT-AS 23 10 616, which is the DT-AS 15 91 300. For active broad-band antennas, which are channel-selective but tuned broad-banded to a frequency band such as, for example the VHF radio frequency range, it is necessary to transform the antenna impedance ZS(f) of a short emitter, into ZA(f) to within the proximity of Zopt. For both electrically large and small antennas, this leads to a frequency-dependent no-load (or open-circuit) voltage on the transistor input. This is expressed by a strongly frequency-dependent effective length le(f) of the passive component of the antenna. In conjunction with the frequency-dependency of the voltage division factor between Zopt, and the input resistance of the transistor deviating therefrom, the resulting frequency curve on the load resistor ZL is flattened with the help of an adapter circuit on the output of the active circuit. This is also required to protect the receiving system, connected downstream, against non-linear effects due to level overloading.
2. Description of the Prior Art
The basic form of an antenna of this type is known, for example from DT-AS 23 10 616, which is the DT-AS 15 91 300. Antennas according to the state of the art are largely mounted, for example, above the high-frequency range, with antenna arrangements installed in a motor vehicle glass pane together with the heating field for heating the window glass pane. This is described, for example in European patents EP 0 396 033, EP 0 346 591, and EP 0 269 723. The structures of the heating field, which are employed to act as the passive antenna component, are components of the vehicle that were not originally meant to be used as antennas. These components of the vehicle can be changed only in minor ways, because of their function for heating purposes. If an active antenna of the state of the art is designed on this type of antenna element, the impedance present on the heating field has be transformed with the help of a primary adapter circuit to within the proximity of the optimum impedance Zopt for noise adaptation, and the frequency curve of the active antenna has to be flattened out with the help of an adapter network located on the output side. This procedure conditions the relatively complicated design of two filter circuits. An advantageous overall performance of the active antenna cannot be accomplished separately for each filter because of the mutual dependency of the two filter circuits. In addition, the amplifier circuit for achieving adequate linearity properties cannot be provided as a simple amplifying element. This narrows the design freedom for the two adapter networks in a noticeable way. In addition, the use of two filters is connected with a substantial expenditure. A further drawback of an active antenna of this type is reflected by the load of the adapter circuit. It has an amplifier connected downstream to the heating field, if a plurality of active antennas are designed from the same heating field to either form an antenna diversity system, or a group antenna with special directional properties, or other purposes. This disadvantageous design exists with all antenna systems in which the passive antenna components are subjected to noticeable cross-coupling of electromagnetic radiation in relation to one another. For example, according to the state of the art, with a multi-antenna scanning diversity system formed from the heating field, switching diodes are mounted at the connection points formed on the heating field for the antenna amplifiers. Each of these switching diodes switches only the adapter circuit with an amplifier, whose signal is switched through to the receiver, and switches the other connection points free. This leads to systems of considerable expense, as well as to the additional requirement that the diodes have to be switched in exact synchronism with the antenna selection.
Accordingly, the present invention provides an active broad-band reception antenna with a specified passive antenna component that has a largely freely selectable frequency dependency of the received signals, independent of the frequency dependency of the effective length and impedance of the passive antenna component, while securing a high sensitivity to noise. For multi-antenna systems, there is a multiple de-coupling of the received signals from a passive antenna arrangement, with a plurality of connection points that are coupled with each other by electromagnetic radiation, without having the received signals mutually influence each other due to the formation of the active antennas.
The antenna system of the invention reduces design costs, and uses simpler circuitry to obtain a reception signal that is optimal with respect to the signal-to-noise ratio, and the risk posed by non-linear effects. Due to the fact that a primary adapter network is omitted, in conjunction with high resistance of the amplifier circuit on the input side, one can design a complicated multi-antenna systems in which the passive antenna components are coupled to each other by radiation. Accordingly, the switching diodes mentioned above are not required in connection with the diversity system for the purpose of switching free connection points where no signal is used for switching through to the receiver.
Other objects and features of the present invention will become apparent from the following detailed description considered in connection with the accompanying drawings which disclose several embodiments of the present invention. It should be understood, however, that the drawings are designed for the purpose of illustration only and not as a definition of the limits of the invention.